With a Web-wide protest on Wednesday that includes a 24-hour shutdown of the English-language Wikipedia, the legislative battle over two Internet piracy bills has reached an extraordinary moment — a political coming of age for a relatively young and disorganized industry that has largely steered clear of lobbying and other political games in Washington.

The bills, the Stop Online Piracy Act in the House and the Protect IP Act in the Senate, are backed by major media companies and are mostly intended to curtail the illegal downloading and streaming of TV shows and movies online. But the tech industry fears that, among other things, they will give media companies too much power to shut down sites that they say are abusing copyrights.

The legislation has jolted technology leaders, venture capitalists and entrepreneurs, who are not accustomed to having their free-wheeling online world come under attack.

One response is Wednesday’s protest, which directs anyone visiting Google and many other Web sites to pages detailing the tech industry’s opposition to the bills. Wikipedia, run by a nonprofit organization, is going further than most sites by actually taking material offline — no doubt causing panic among countless students who have a paper due.

It said the move was meant to spark greater public opposition to the bills, which could restrict its freedom to publish.

“For the first time, it’s very clear that legislation could have a direct impact on the industry’s ability to do business,” said Jessica Lawrence, the managing director of New York Tech Meetup, a trade organization with 20,000 members that has organized a protest rally in Manhattan on Wednesday. “This has been a wake-up call.”

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Tim Wu, a professor at Columbia Law School, said that the technology industry, which has birthed large businesses like Google, Facebook and eBay, is much more powerful than it used to be.

“This is the first real test of the political strength of the Web, and regardless of how things go, they are no longer a pushover,” said Professor Wu, who is the author of “The Master Switch: The Rise and Fall of Information Empires.” He added, “The Web taking a stand against one of the most powerful lobbyers and seeming to get somewhere is definitely a first.”

Under the proposed legislation, if a copyright holder like Warner Brothers discovers that a foreign site is focused on offering illegal copies of songs or movies, it could seek a court order that would require search engines like Google to remove links to the site and require advertising companies to cut off payments to it.

Internet companies fear that because the definitions of terms like “search engine” are so broad in the legislation, Web sites big and small could be responsible for monitoring all material on their pages for potential violations — an expensive and complex challenge.

They say they support current law, which requires Web sites with copyright-infringing content to take it down if copyright holders ask them to, leaving the rest of the site intact. Google, which owns YouTube and other sites, received five million requests to remove content or links last year, and it says it acts in less than six hours if it determines that the request is legitimate.

The major players supporting the legislation, including the United States Chamber of Commerce and the Motion Picture Association of America, say those measures are not enough to protect intellectual property. They emphasize that their primary targets are foreign Web sites that sell counterfeit goods and let people stream and download music and video at no charge — sites that are now largely out of reach of United States law enforcement. And they are fighting against what they characterize as gimmicks and distortions by Internet companies opposed to the bills.

With talk of censorship and loss of Internet freedom, “the current debate has nothing to do with the substance of the bills,” David Hirschmann, who leads the Chamber of Commerce’s initiative on intellectual property, said in an interview. “We will certainly use every tool in our toolbox to make sure members of Congress know what’s in these bills.”

With financial resources that few other groups can match, the chamber is one of Washington’s most powerful lobbying forces and has shown the ability to alter Congressional debate on its own.

Senator Patrick J. Leahy, Democrat of Vermont and author the Protect IP Act, accused opponents Tuesday of trying to “stoke fear” through tactics like the Wikipedia blackout. “Protecting foreign criminals from liability rather than protecting American copyright holders and intellectual property developers is irresponsible, will cost American jobs, and is just wrong,” he said in a statement.

Opponents of the legislation have clearly seized the momentum in the debate. Their protests have gained traction in that key provisions were stripped out of one bill and the Obama administration has raised concerns. Legislators have already agreed to delay or drop one ire-inducing component of the bills, Domain Name System blocking, which would prevent access to sites that were found to have illegal content.

A total of 115 companies and organizations have lobbyists working on the antipiracy bills, spending millions of dollars to sway the outcome, according to federal disclosure records. They include corporate and technology giants on both sides of the legislation, with entertainment groups like News Corporation and the Recording Industry Association of America backing it and Internet firms like Google and Facebook raising concerns about it.

The largest advocates for the bills disagree with the tech industry’s main rallying cry, which is the notion that they will hurt the average Internet user or interfere with their online activities.

“The bill will not harm Wikipedia, domestic blogs or social network sites,” said Representative Lamar Smith, Republican of Texas and a primary sponsor of the House bill.

Most people in the tech world agree that the problem of piracy needs to be addressed. But they say their main concern is that the tech industry had little influence on the language of the legislation, which is still in flux and so broadly worded that it is not entirely clear how Internet businesses will be affected. Big Internet companies say the bills could prevent entire Web sites from appearing in search results — even if the sites operate legally and most content creators want their videos or music to appear there.

“It shouldn’t apply to U.S. Web sites, but any company with a server overseas or a domain name overseas could be at risk,” said Andrew McLaughlin, vice president at Tumblr, a popular blogging service.

Mr. McLaughlin said the fear is that on large and diverse Web communities like Tumblr, any user who uploads an unauthorized clip from a movie or an unreleased track from an album is putting the whole company in the line of fire.

In November, Tumblr rigged a tool that “censored” the page its users see when they log into the site, explained the legislation and routed them to contact information for their representatives in Congress. The stunt resulted in 80,000 calls to legislators in a three-day period. Mr. McLaughlin said the company was planning a similar approach for Wednesday.

Some who oppose the bill, including the Electronic Frontier Foundation, an online rights group, see a bright spot in a potential compromise called the OPEN Act, which would provide for the International Trade Commission to judge cases of copyright or trademark infringement. If the commission found that a foreign site was largely devoted to piracy, it could compel payment processors and online advertising companies to stop doing business with it.

Silicon Valley has championed companies that provide alternatives to piracy, like Spotify and Netflix. And the industry says that the problem could be solved by letting it do what it does best — innovating.

“It’s something that could be solved using technology through collaboration with these start-ups,” said Ms. Lawrence of New York Tech Meetup.

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